Secondly and thirdly, Bignonia unguis with its close allies, and
Cardiospermum; but their tendrils are so short that their contraction
could hardly occur, and would be quite superfluous. Fourthly, Smilax
aspera offers a more marked exception, as its tendrils are moderately
long. The tendrils of Dicentra, whilst the plant is young, are short
and after attachment only become slightly flexuous; in older plants
they are longer and then they contract spirally. I have seen no
other exceptions to the rule that tendrils, after clasping with their
extremities a support, undergo spiral contraction. When, however,
the tendril of a plant of which the stem is immovably fixed, catches
some fixed object, it does not contract, simply because it cannot;
this, however, rarely occurs. In the common Pea the lateral branches
alone contract, and not the central stem; and with most plants, such
as the Vine, Passiflora, Bryony, the basal portion never forms a
spire.
I have said that in Corydalis claviculata the end of the leaf or
tendril (for this part may be indifferently so called) does not
contract into a spire. The branchlets, however, after they have
wound round thin twigs, become deeply sinuous or zigzag.
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